The present invention relates to a system of colorants permitting the attainment of tinted, sodium fluoride opal glasses and, more particularly, opal glasses whose tint lies in the shades ranging from "cream-white" or "ivory" to "beige".
Opal glasses have been utilized for a long time in the fabrication of culinary articles (tableware, baking ware), the purchase of which is strongly influenced by their aesthetic appearance which is generally obtained on the one hand by the coloration in the body of the opal glass and, on the other hand, by the application of decorations added to the article after shaping. Where opal glass is concerned, a range of particularly attractive tints extends from the color "cream-white" to "ivory" and "chestnut-beige". In the lightest version (cream-white), the desired tint is notably different from that of the white opal glass products available today, being, more particularly, closer to that of certain articles of porcelain, as, for example, the porcelain known under the name of "WEDGEWOOD BONE CHINA". Of all the range, this light tint is the one which is most in demand.
The invention, which is aimed at procuring such tinted opal glasses, is concerned more especially with the manner of obtaining those tints through the use of sulfur or compounds thereof in the presence of iron in compositions of opal glass whose principal crystal phase is sodium fluoride.
The production of colored glasses through the aid of inorganic colorants has been known for a long time. The book by W. A. Weyl entitled "Coloured Glasses", published by The Society of Glass Technology, Sheffield, England (1951) describes extensively the colors produced by certain ions and suggests mechanisms leading to the coloration. More particularly, it describes the coloration of "transparent" glasses by means of iron oxide and sulfur or sulfates prepared in a reducing environment for obtaining, among others, the glasses called "ambers". The glasses generally belong to the Na.sub.2 O-CaO-SiO.sub.2 system. It is generally accepted that the coloration results from centers formed through a ferric ion (Fe.sup.+3) in tetrahedral coordination with three oxygens and a sulfide ion (S.sup.-2), electrical neutrality being assured by the presence of a cation such as Na.sup.+, for example.
The introduction of sulfur in the form of sulfides instead of sulfates or of elemental sulfur into soda lime glasses is, according to the literature (R. D. Wought, Glass Technology, 23, No. 6, page 259, 1982) conducive to obtaining a tint that is stable in production; melting always takes place in the presence of a highly reducing compound such as carbon.
The transposition to opal glasses of the colors obtained with certain colorant systems in transparent glasses does not always occur. For example, the use of NiO, which permits yellow and brown tints to be obtained in transparent glasses, leads to the development of gray or violet tints in the opal glasses with which this invention is concerned.
Systems of colorants appropriate to opal glasses are known, however. In particular, British Patent No. 2,077,716 is concerned with the use of the oxides of nickel, cerium, and titanium in opal glasses in which the opacifying phase is calcium fluoride, to obtain colors similar to those of the present research. Nevertheless, those glasses exhibit insufficient opacity to satisfy the new needs of the market.
French Patent No. 1,531,756 describes the production of "ivory" opal glasses from the colorant mixture (Fe.sub.2 O.sub.3 +MnO.sub.2) with or without TiO.sub.2. With this system of coloration for dark tints in similar opal glasses, it appears that, when used with articles pressed and then annealed, a non-uniform brown color generally develops on the surface which is unacceptable. The amounts of Fe.sub.2 O.sub.3 and MnO.sub.2 utilized were 0.70% and 1.25% by weight, respectively.
It appears that no opal glasses exist whose principal crystal phase is sodium fluoride tinted by the iron-sulfur in the presence or not of a reducing agent.